Inhibitory (GABAergic) neurons constitute about 15-30 percent of cells in the neocortex and are classified into distinct groups according to their morphology and histochemistry. However, the functional significance of this diversity and the characteristics of inhibitory interactions of specific types of GABAergic neurons in terms of local circuit organization and the processing of information in the neocortex are only poorly understood. We have developed methods allowing us to identify and study interactions of specific types of inhibitory neurons. Using these methods we have shown that a prominent class of inhibitory neurons, the fast-spiking (FS) cells, is specifically and extensively interconnected via electrical synapses. Furthermore, we have found that the FS cell network can produce synchronous spiking and can be sensitive to coincident activity. In preliminary studies we have found extensive electrical synapses among another class of cells located in layer 1 termed late-spiking (LS) cells. Together, these data lead us to hypothesize that specific synaptic connections of different classes of inhibitory neurons form GABAergic networks that coordinate activity in the neocortex. The long-term goals of this proposal are to study how different classes of inhibitory interneurons are organized in terms of their specific synaptic interactions and to study the functional implications of these interactions for the processing of information in the visual cortex. We propose to address the following three specific aims: 1. Physiological properties of GABAergic synapses made by FS cells and LS cells. 2. Properties of electrical synapses among FS cells and among LS cells. 3. Emergent properties of networks formed by FS cells and LS cells.